Methods of manufacturing and handling target electrodes and target electrode assemblies



y 8, 1962 H. J. HANNAM 3,032,859

METHODS OF MANUFACTURING AND HANDLING TARGET ELECTRODES AND TARGETELECTRODE ASSEMBLIES Original Filed May 23, 1958 2 Sheets-Sheet lINVENTOR:

HERBERT J H:NNAM, mgr

HIS TORNE May 8, 1962 H. J. HANNAM 3,032,859

METHODS OF MANUFACTURING AND HANDLING TARGET ELECTRODES AND TARGETELECTRODE ASSEMBLIES Original Filed May 25, 1958 2 Sheets-Sheet 2INVENTORI HERBERT HANNAM,

HIS TORNEY ite Sat My invention relates to an improved target electrodeassembly and more particularly to an improved assembly of this type forproducing a point-by-point electric charge pattern corresponding to avisual image or other information to be converted to electrical signalsby scanning the target electrode with an electron beam. Additionally, myinvention relates to storage electrodes employable in target electrodeassemblies and improved methods of manufacturing and handling suchelectrodes and assemblies. This is a divisional application of mycopending US. application Serial No. 737,348, filed May 23, 1958, andissued as US. Patent No. 2,922,907 on January 26, 1960.

In US. patent application Serial No. 630,683 of Harold R. Day, Iri, andPeter Wargo, filed December 26, 1956, entitled Target Electrode Assemblyand assigned to the same assignee as the present application there isdisclosed and claimed a target electrode assembly adapted for use, forexample, in a known type of television camera tube, referred to as animage orthicon. This assembly includes a thin transparent film ofmagnesium oxide and a conductive mesh supported from opposite sides of arela tively rigid glass mesh structure having a large number of closelyspaced openings extending therethrough generally perpendicular to thefilm. This structure is mechanically rigid for minimizing undesirablemechanical vibrations which could result in microphonics or unwantedelectrical signal modulations. Additionally, this structure is adaptedfor maintaining relatively constant electrical characteristics over longperiods of use. It is considered that the conduction in a directionnormal to the opposed faces of the magnesium oxide film is due to thetransport through the film of electrons rather than ions, and there isno loss of available electrons in the film after long periods of usecorresponding to the decrease in available mobile ions occurring inpreviously employable glass films. Thus this type of assembly is notsubject to the undesirable phenomenom which has been called burn-in andwhich results in a tendency for an after image to be retained on theelectrode for a period many times the frame repetition rate, causing theimage to be superimposed upon later scenes. In operation of thestructure disclosed in the above-referenced application Serial No.630,683, a charge pattern is established on the magnesium oxide film inaccordance with the secondary electrons emitted from one surface of thefilm in response to impingement upon the opposite surface thereof ofelectrons from a photocathode. Inasmuch as the magnesium oxide filmprovides a high yield of secondary electrons, it results in a sensitivetarget electrode.

The present invention contemplates an improved target electrode assemblywhich will afford all of the advantages obtainable with theabove-described assembly of Day and Wargo without reliance on anintermediate electrode support and which affords substantial furtheradvantages.

Accordingly, it is a primary object of my invention to provide a new andimproved target electrode assembly which maintains its electricalcharacteristics over long periods of use.

Another object of my invention is to provide a new and improved targetelectrode assembly wherein a void is provided between the spacedelectrodes thereof and thus is not subject to spurious signals due tocharge accumulaatnt tions on and leakages across elements interposedbetween the spaced electrodes.

, Another object of my invention is to provide a new and improved targetelectrode assembly including new and improved means for avoidingundesirable mechanical vibrations and resulting unwanted electricalsignal modulations Without reliance on support means disposed within thearea defined by the margin of the assembly.

Another object of my invention is to provide a new and improved storageelectrode adapted for improved image resolution.

Another object of my invention is to provide a new and improved targetelectrode assembly adapted for increased electron transmission and thusadapted for higher sensitivity.

Another object of my invention is to provide a storage electrode whichrequires support only at its periphery and, thus, is capable of highersecondary emission than storage electrodes which require supportstructure across the surface thereof and which are subject to reducedsecondary emission resulting from the adverse effects of foreignmaterials evolving from the support structure.

Another object .of my invention is to provide an improvide targetelectrode assembly which comprises a reduced number of essentialelements and thus is adapted for reducing the costs and efforts ofmanufacture.

Another object of my invention is to provide an improved storageelectrode which overcomes the need for utilizing elements formed ofmaterials such as glass which generally add to the difiiculties ofevacuating envelope structures adapted for containing such targetelectrodes.

Another object of my invention is to provide improved methods ofmanufacturing and handling target electrode assemblies and storageelectrodes therefor adapted for facilitating and reducing the cost ofmanufacture.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming part of this specification.

In carrying out the objects of my invention I provide a target electrodeassembly including a conductive mesh electrode and an annular electrodesupport corresponding generally to the margin of the mesh electrode. EX-tending tautly across the annular electrode support and supported solelythereby is a membrane of polycrystalline homogeneous magnesium oxide.The membrane is spaced at predetermined distance from the mesh electrodeand has a thickness of substantially the same order of magnitude as thesize of the crystals constituting the membrane. The membrane is of amass per unit area which requires for excitation a vibrational frequencyof such a high magnitude as to be practically vibrationless in ordinarytarget electrode environments. Furthermore, the resonant frequency issuch as to be almost undetectable at normal frame and scanning rates.The target electrode is manufactured by first forming a vaporizablesupport film on an annular support member evaporating a magnesiumcoating on the film and then oxidizing the mag nesium and vaporizing thesupport film, thereby to leave a taut magnesium oxide membrane supportedonly by the annular support. The magnesium oxide membrane is frail orsubject to easy breakage and destruction of the membrane by relative airmovement is avoided by maintaining an air breaking element incloselyspaced relation with the membrane during manufacturing and assemblymovements thereof.

For a better understanding of my invention reference may be had to theaccompanying drawing wherein:

FIG. 1 is an elevational view in section, schematically representing acamera tube of the type to which the present invention may be applied;

FIG. 2 is an enlarged elevational view in section, illustrating theelectrode assembly, including the target electrode, of the image sectionof tube shown in FIG. 1;

FIG. 3 is a perspective view showing the construction of the targetelectrode assembly of the present invention;

FIG. 4 is a perspective fragmentary view, partially in section andgreatly enlarged, showing constructional details of the target electrodeassembly of the present invention;

FIG. is an enlarged fragmentary sectional view illustrating a step inone method of manufacturing the target electrode of the presentinvention; and

FIG. 6 is an enlarged fragmentary sectional view illustrating a step inanother method of manufacturing the target electrode of the presentinvention.

As best seen in FIG. 4, the target electrode assembly of the illustratedembodiment of the present invention, includes planar eleotronpermeableelectrode generally designated 1 and a storage electrode generallydesignated 2 including a transparent magnesium oxide membrane 3supported in spaced relation to the electron-permeable portion of theplanar electrode 1.

The electrode 1 can comprise an electrofo-rmed mesh 4 which has beenalum-inized on both sides and is mounted on an annular support ring 5.The support ring 5 includes an annular channel 6 in which is received aretaining ring 7 adapted for securely retaining the edge of the mesh 4in the channel 6 and, thus, securing the mesh to the support ring 5.

The magnesium oxide membrane 3 of the storage electrode 2 is supportedsolely by an annular support 8 which is formed to correspond generallyto the outer edge or margin of the mesh supporting ring 5 of theelectrode 1. The support 8 has the membrane 3 directly mounted on theundersurface thereof in FIG. 4 and is suitably secured, as by brazing,to a ring 9 which is of angular cross-section and thus is adapted forrigidizing the ring 8. An annular spacer 10 interposed between theelements 5 and 8 determines the spacing between the mesh 4 and themembrane 3. Thus, the mesh 4 and the membrane 3 are adapted for beingmaintained in a spaced relation or are separated by a void which extendscompletely across the corresponding areas of these elements. Preferablythe spacing can be from approximately .5 to 150 mils. For monochrometelevision purposes a spacing of 2.5 to 3 mils is highly satisfactory,While a spacing of approximately .5 mil works well for color televisioncamera tubes. The spacing between the mesh and membrane determines thetime constant of the assembly and it will be understood from theforegoing that this time constant can be varied by varying the spacingwithin the limits of the range noted.

The substance of the magnesium oxide membrane 3 is highly flimsy orfrail in form and requires particular methods of manufacture andhandling to obtain and sustain the formation of a membrane of such asubstantial area as that defined by and extending otherwise unsupportedacross the annular membrane support 8. In accordance with one preferredmethod of forming the membrane 3, a suitable thin layer ofnitrocellulose illustrated at ll in FIG. 5 is applied to the membranesupport 3 by first dropping onto the surface of a pan of water a smallquantity of nitrocellulose dissolved in a suitable organic solvent suchas amyl acetate. This solution spreads out into a thin film due tosurface tension and the solvent evaporates, leaving a plastic film onthe surface of the water. Thereafter, the membrane support ring 8, whichhas been placed in the water either prior to the formation of the filmor which is immersed in the water at the outer portion of the film, israised gently to pick up the film on the surface of the ring.

After the film has dried completely, the ring is placed in an evaporatorand a thin coating of magnesium shown at 1?. in FIG. 5 is evaporated onthe plastic film to a de sired thickness. The thickness of the magnesiumthus evaporated on the film is determined by the desired mechanical andelectrical characteristics of the storage electrode. In the particularembodiment illustrated, the. film of magnesium is approximately 500angstroms thick.

Thereafter, the electron-permeable electrode 1 and the just-describedtarget electrode structure are assembled in the manner shown in FIG. 5and the whole assembly is placed in an oven and baked out in air,starting at a temperature of about 170 C. and terminating at about 400centigrade for a period in the order of approximately five hours. Thisbaking decomposes and vaporizes the nitrocellulose film which disappearscompletely and also is etfective for reducing the magnesium to an oxidefor thus forming a smooth, taut, transparent, colorless magnesium oxidefilmor membrane. During the baking operation and subsequently duringhandling of the assembly the magnesium oxide membrane, which is veryflimsy or frail, is prevented from being destroyed by air movementrelative thereto by the mesh 4 of the electrode 1 which serves as an airbreak or air baffling element. In view of the fact. that the mesh 4 isincluded in the assembly which is subjected to the air bake it isessential that the mesh 4 be capable of withstanding the air bakingoperation Without oxidizing.

With a magnesium oxide membrane thickness of 500 angstroms, the timeconstant of the storage electrode structure is suitable for a repetitionrate of 30 frames persecond used in television. The time constantincreases as the thickness of the magnesium oxide membrane increases andinformation storage may be realized with magnesium oxide films of athickness in the order of several thousands of angstroms.

In membrane 3 the conduction is electronic. That is, there is noreliance on ions which in prior art glass membranes has a tendency tobecome depleted and, therefore, the target conductivity in the presentstructure is stable and burn-in is eliminated. By the use of X-raydefraction patterns I have learned that the magnesium oxide membraneobtained by the above-described method is homogeneous andpolycrystalline and that the crystal size is about 300 angstroms. Thus,in a membrane having a thickness of the order of 500 angstroms themembrane thicknesses is of substantially the same order of magnitude asthe size of crystals constituting the membrane; and therefore,conductivity through the film by grain boundary conduction orbombardment induced conductivity is enhanced. This type of conductionenables the utilization of magnesium oxide as a membrane material eventhough it has a much higher resistivity than glass formerly used. Thishigh resistivity results in extremely low lateral leakage which can berelied upon to improve image resolution.

In considering the substantially large area of the magnesium oxidemembrane 3, it might be felt that the assembly would be subject tomicrophonics due to drum head vibrations of the membrane. However, inthe assembly of the present invention microphonics are substantiallyreduced due to the fact that the mass per unit area of the membrane 3 isabout times less than glass membranes formerly employed. Thus, thevibrational frequency of the membrane 3 is approximately 10 times ashigh as for a glass membrane when those types of membranes are under thesame tension. Additionally, the membrane 3 can be placed under greatertension per unit cross section than can prior art glass membranes. Thevibrational frequency of the membrane 3 is more difficult to excite andeven if excited image variations resulting therefrom are not generallydiscernible at normal scanning rates and thus are not generallyobjectionable. For all practical purposes the membrane 3 might beconsidered vibrationless in ordinary target electrode environments.

Thus, it will be seen that the assembly of FIGS. 3 and 4 is adapted,since it operates by means of electron conduction rather than ionconduction for maintaining its electrical characteristics over longperiods of use. Ad-

ditionally, the membrane 3 is effective for avoiding the undesirablemechanical vibrations and resultant microphonics without reliance on anintermediate support such as the glass mesh disclosed in applicationSerial No. 630,683. This has the desired effect of eliminating the needfor the glass mesh which is currently a substantially expensive anddiflicult to manufacture element. Additionally, by providing a structureincluding a void between the mesh and membrane I have reduced thepossibilities of spurious signals due to charge accumulations andleakages across portions of material extending between the electrodes.Due to the void between the elements electron transmission through theassembly is increased, thereby to afford improved image resolution.Also, increased electron transmission adapts the assembly for a highersensitivity. Further, in structures utilizing a membrane supportextending across the surface of the membrane secondary emission can bereduced substantially due to foreign matter evolving from the materialof the support. Applicants membrane 3 does not require this type ofsupport and, thus, is adapted for desirably greater secondary emission.Still further, glass is noted as a material which desorbs vapor uponheating and the elimination of glass from the target assembly serves notonly to eliminate the need for a relatively expensive element and tosimplify the structure and effort of manufacture, but also serves toremove from a tube another element which might constitute a source ofdifficulty in evacuating an envelope containing the assembly.

Alternatively, the assembly of FIG. 3 can be obtained by forming avaporizable film 11 on the annular ring 8, then coating the film withmagnesium in the manner illustrated at 12 in FIG. 6, and then air bakingthis assembly while supported, for example, on a plurality ofcircumferentially spaced upstanding elements 13. Thus, the vaporizablefilm 11 is decomposed and the magnesium coating 12 is reduced to amagnesium oxide membrane 3 which extends completely across the member 8and is solely supported thereby. When the storage electrode is formed inthis manner it is necessary to transport it for assembly with theelectron permeable electrode 1 and it is necessary to accomplish thiswithout fracture or destruction of the membrane 3. To accomplish this Iplace a plate 14 over the target electrode 2 and in engagement with therim of the element 9 and by con-currently grasping the contiguous edgesof these elements it is possible to lift both and transport the targetelectrode to a position Where it can be assembled to the electronpermeable electrode 1 with the element 14 acting as an air break or airbaffling means, thus to avoid fracture or destruction of the membrane bymovement of air relative to the support 8 therefor. After the targetelectrode is assembled to provide the target electrode-electronpermeable electrode assembly, the mesh 4 of the electrode 1 can serve toavoid destruction of the membrane 3 during movement of the targetelectrode assembly for mounting in a camera tube.

As best seen in FIGS. 2, 3, and 4 the mesh-support ring 5 has securedthereto a plurality of angularly spaced brackets 15 which carryrotatable resilient tabs 16. The tabs 16 are adapted for being movedinto the radially inwardly projecting positions thereof illustrated inFIGS. 3 and 4 wherein they are effective for retaining the targetelectrode 2 in assembled relation with respect to the electrode 1.Additionally, the support 5 carries a plurality of angularly spaced andlaterally slotted mounting tab 17 which are adapted for mounting theassembly in a camera tube in the manner illustrated in FIG. 2. In thearrangement of FIG. 2 the target electrode assembly is supportedtransversely in a cylindrical target supporting electrode 20 by means ofthe electrode 20 by means of the slotted mounting tabs 17 being fittedabout suitable holding bolts 21 carried by an inturned flange 22 formedon the cylinder 20. Thus, the target electrode assembly is supportedopposite the opening in a cylindrical flange 23 comprising part of thetarget assembly supporting electrode 20. The latter electrode forms partof the assembly including an accelerating grid electrode 24 and adecelerating grid electrode 25. These three electrodes are supported inlongitudinally spaced coaxial relation by suitable ceramic rods orstalks 26 spaced around the circumference of the electrodes and heldthereto by suitable straps 27. This assembly is supported in theenlarged image section of the tube shown schematically in FIG. 1 withthe accelerating grid electrode 24 spaced slightly from a photocathode28 which provides a source of photoelectrons. The photoelectrons areaccelerated toward the target electrode assembly to establish'a chargepattern thereon in accordance with the image falling on thephotocathode. At the opposite end of the tube is the electron gun andelectron multiplier structure which are concentrically arranged. Thegun, which provides the scanning beam, is shown merely as a hollowcylindrical grid electrode 30, having a small aperture 31 in the orderof .002 inch in diameter in the end wall thereof, for producing a thinscanning beam. The outer surface of this end wall surrounding theaperture also provides the first dynode of the electron multiplier, aswill be described in more detail hereinafter. A cylindrical electrodewhich may be formed as a metallic coating 32 on the neck of the tubeprovides for focusing the beam and the field controlling electrode 25,usually designated a decelerating electrode. As will be readilyappreciated by those skilled in the art, the entire camera tube issubjected to an essentially homogeneous longitudinal collimatingmagnetic field. This field may have a strength of gauss, for example.Electrons from the scanning beam are collected in accordance with thecharge or potential pattern established on the target electrode so thatreturned electrons, which are the forward beam electrons minus thosecollected, vary with the charge pattern on the target electrode 3. Theseelectrons do not reenter the aperture 31 but instead strike the platesurrounding the aperture, which is a high secondary emitter so thatthere results a multiplication of the electrons emitted compared withthose returned from the storage electrode.

A generally cyindrical focusing electrode 33 for the electron multipliersection of the tube is supported at the end of the gun electrode 30intermediate that electrode and the beam focusing grid electrode 32.

Several stages of electron multiplication are provided by electrodes3437 inclusive and the amplified electron current is collected by theanode 38 of the electron multiplier to produce a signal across theresistor 39 which varies in accordance with the charge pattern on themembrane 3. In FIG. 1 of the drawing, suitable direct current operatingvoltages for the various electrodes have been indicated. These voltagesare relative to the cathode and may vary appreciably from the valuesgiven.

When the target electrode is scanned by an electron beam, the variationsin beam current collected by the anode 38 reproduces point-by-point anelectrical signal varying in accordance with a charge pattern on thetarget electrode. The time constant of the membrane 3 determines theframe speed on which the device operates, hence it is essential that theresidual charges from one frame to another be so small as not tointerfere with the production of an electrical signal indicative of theimage falling on the photocathode in any particular frame.

With the magnesium oxide membrane 3, the electrical characteristicsremain relatively constant over extended life of the membrane. Theundesirable burn-in phenomenon resulting from what is generallyunderstood to be a depletion of the mobile ions in glass membranes isnon-existent. The magnesium oxide membrane 3 provides a target which isavailable on both faces for the impingement of the electron. Also, thestructure of the present invention provides for relatively vibrationlesstarget electrode which is substantially free of undesirable microphonicsdue to target electrode movement. Addi- T? tionally, in the presentstructure no intermediate electrode support means is required whichaifords improved resolution, higher sensitivity, higher secondaryemission, freedom from certain spurious signals, and reductions in costsand efforts in manufacturing the electrode assembly as well as .anevacuated device incorporating same.

While I have shown and described a particular embodiment of myinvention, 1 do not desire my invention to be limited to the particularform shown and described, and I intend by the appended claims to coverall modifications within the scope of my invention.

What I claim as new and desire to secure by Letter Patent of the UnitedStates is:

1. The method of manufacturing and handling a storage electrodecomprising the steps of forming a vaporizable film across an annularsupport having a transverse dimension comparable to that of a finishedelectrode, evaporating a magnesium coating on said film, heating theassembly in an oxidizing atmosphere. to both decompose said film andconvert said magnesium coating to a taut, frail magnesium oxide membraneextending completely across and being solely supported by said annularsupport, and maintaining an air. baflling element, in spaced relationwith said membrane, thereby to avoid destruction of said membrane byrelative movement. of

air.

2. The method of processing a target electrode as sembly comprising thesteps of forming a vaporizable film across an annular supportapproximately the. size of a finished assembly, evaporating a magnesiumcoating on said fi securing said support with said film thereon to amesh electrode with said mesh electrode disposed in spaced confrontingair bafiling relation to said magnesium coating, air baking the assemblyto both decompose said film and convert said magnesium coating to ataut, frail discrete magnesium oxide storage membrane which extendscompletely across said annular support in spaced relation to said meshand is solely supported by said, annular support, with said meshelectrode acting as air baffling means for said membrane.

3. The method of manufacturing a storage target electrode comprising thesteps of forming a vaporizable film across an annular support memberhaving a dimension comparable to that of a finished electrode,evaporating a magnesium coating on said film, heating the assembly in anoxidizing atmosphere for a, period in the order of approximately 5 hoursstarting at a temperature of about 170 C. and, terminating at, about 400C. to decompose said film and oxidizing said magnesium coating to ataut, homogeneous polycrystalline magnesium oxide membrane extendingcompletely across and being solely supported by said annular support.

References Cited in the tile of this patent UNITED STATES PATENTS

1. THE METHOD OF MANUFACTURING AND HANDLING A STORAGE ELECTRODE COMPRISING THE STEPS OF FORMING A VAPORIZABLE FILM ACROSS AN ANNULAR SUPPORT HAVING A TRANSVERSE DIMENSION COMPARABLE TO THAT OF A FINISHER ELECTRODE, EVAPORATING A MAGNESIUM COATING ON SAID, HEATING 